Ethiopia is mainly a cash society. In most of the places we will visit you will not be able to use a credit card.
When I visited last year I tried a number of ATMs - but none of them worked. I ended up using cash for everything. I changed my currency at the hotel and kept the cash in the hotel safe and carried small amounts of cash with me. I had no problems changing dollars, Pounds or Euros.
There are lots of banks around - and will be able to change money in the different hotels. It is best not to change more than you need - as it might not be easy to change back at the end.
I also found things much cheaper in Ethiopia than I expected - so money lasted longer than anticipated